This invention relates to a document transporting device and method for use with copiers, image scanners, facsimile machines and the like, for successively transporting a set of documents to enable image data reading on both sides of each of the documents, and an image reading device provided with the same.
FIGS. 6A-6E show an example of an image reading device and an image reading operation of prior art. As shown in FIG. 6A, this image reading device is constructed in such a manner that a set of documents stacked on a document tray 10 are fed one by one by a document separator/feeder 20 onto a predetermined image reading position 71 of an image reading unit 70 to enable image data reading on one side (corresponding to a side facing upward on the document tray 10) of the document. In the case of double sided image reading operation, after passing the image reading position 71, the document is transported to a document inverting unit 60 where the document is turned upside down while having the transport direction thereof inverted along a switchback path (inverting path) provided therein. After turning the document side in the document inverting unit 60, the document is transported to the image reading position 71 again to enable image data reading on the opposite side of the document that faces downward on the image reading unit 70 this time.
Let us illustrate procedures of double sided image reading operation performed by the prior art image reading device with graphical indications that, in FIG. 6, P.sub.11 denotes the first page of a first sheet, P.sub.12 denotes the second page of a first sheet, . . . , P.sub.n1 denotes the odd number page of n-th sheet (last document), and P.sub.n2 denotes the even number page of the last document.
The above set of documents carrying image data on both sides of each of the documents are generally stacked on the document tray 10 in such a manner that the first page P.sub.11 comes the uppermost position of the document set facing upward, the second page P.sub.12 faces downward, . . . , and the last even number page P.sub.n2 comes the lowermost position facing downward (in other words, they are stacked such that P.sub.11, P.sub.12 P.sub.21, P.sub.22, . . . , P.sub.n1, and P.sub.n2 from the uppermost position in this order as shown in FIGS. 6A and 6B).
The set of documents on the document tray 10 is fed from the uppermost document (P.sub.11 & P.sub.12) for the following reason. Generally, a set of documents stacked on the document tray 10 are separated one by one by a frictional force exerted on the surface of the document in contact with a feeder roller or its equivalent, and fed inside a main body of the image reading device. If the document set is fed from the lowermost document, a relatively large force to press the whole set of documents downward is required to cause the feed roller to assuredly feed in the lowermost document to the device main body. Such a large pressing force increases the frictional force of the document fed by the feed roller. Thereby, it is highly likely that the image of the document being fed by the feed roller is smeared with the image of the document in contact therewith or vice versa. To avoid such a problem, the image reading device is so constructed as to allow a set of documents stacked on the document tray 10 to be fed from the uppermost sheet.
Referring back to the document set stacked on the document tray 10, after the double sided image reading operation according to the document transport procedure carried out by the above image reading device, assuming that there was no further document inverting step, each of the documents is discharged on the discharge tray 80 with the side of the odd number page facing upward and stacked one over another successively (see dotted lines in FIG. 6D). Accordingly, what would have been obtained on the discharge tray 80 at the step as shown in FIG. 6D, assuming that there was no further document inverting step, is the document set stacked in the order of P.sub.n1, P.sub.n2, P.sub.(n-1)1, P.sub.(n-1)2, . . . P.sub.21, P.sub.22, P.sub.11, and P.sub.12 from the uppermost position, the order different from the initial order when stacked on the document tray 10.
To avoid the document set on the discharge tray 80 arranged in the reversed order as the document tray 10, the above image reading device is constructed such that the document after the double sided image reading operation is transported back to the document inverting unit 60 to turn the document side (see FIG. 6D), thereby discharging the documents on the discharge tray 80 in the order that the odd number page of the documents faces downward after passing the image reading position 71 (this time no image reading). Thereby, the document set is discharged on the discharge tray 80 without changing the order of page number (see dotted lines in FIG. 6E), in the same order as the document tray 10.
Summing up the above operation, the above image reading device is operated such that each document passes the image reading position 71 three times (the first is for a one side image reading operation; the second is for an opposite side image reading operation; and the third is for no image reading operation as shown in FIGS. 6B, 6C, and 6D) in total to perform double side image reading and be discharged on the discharge tray 80 in the same order as the document tray 10.
Note that P.sub.IA in FIG. 6B represents the document whose image has not yet been read; P.sub.IB in FIGS. 6B and 6C represents the document whose one side image reading has been done; P.sub.IC in FIG. 6D represents the document whose double sided image reading operation has been done; and P.sub.ID in FIG. 6E represents the document whose orientation on the discharge tray 80 is upside down (flipped over) to the orientation of the document indicated by P.sub.IC shown in FIGS. 6C and 6D.
It should be recognized by now that the last (third) operation is unnecessary so far as the image reading operation is concerned, and it may cause smear on a contact glass at a position corresponding to the image reading position 71 because of this additional passing and lead to a poor image formation.
Further, in the above image reading device, the discharge tray 80 is provided on an extended area of the inverting path provided in the document inverting unit 60. Accordingly, it is highly likely that a lead end (left side in FIG. 6A) of the document on the way of inverting the transport direction along the inverting path (the direction R in FIG. 6A) may push the documents that have already been stacked on the discharge tray 80 without alignment/holding means, thereby resulting in non-alignment of the discharged document set, or in the worst case, sliding off the discharged document(s).
There is an idea of setting the discharge tray 80 vertically at a lower position relative to the inverting path so as not to interfere with the inverting movement of the document, thereby preventing a non-aligned state of the document set on the discharge tray 80 or slipping off of the document(s) from the discharge tray 80. This idea, however, unavoidably enlarges the vertical dimension of the image reading device as a whole.
There is another idea of providing a sub tray between the document tray 10 and the discharge tray 80 to hold the document on the way of inverting the transport direction along the inverting path. Providing such a sub tray, however, also enlarges the vertical dimension of the image reading device as a whole. Furthermore, when taking out the document set from the discharge tray 80, a user must lift up the document tray 10 and the sub tray to access to the discharge tray 80, which is not feasible in the aspect of operability of the image reading device.